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Top quark mass For DØ collaboration Regina Demina University of Rochester Wine and Cheese seminar at FNAL, 07/22/05 Outline • Introduction • Top quark mass measurement in Run II – – – – – Matrix element method description In situ jet energy scale calibration on hadronic W-mass Sample composition Result Systematics • Tevatron combined top mass • Top quark production – Update on cross section in l+jets channel – Search for resonance production 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 2 Top Quark Mass: Motivation • Fundamental parameter of the Standard Model. • Important ingredient for EW precision analyses at the quantum level: t W CDF&D0 RUNII H W W W b MW mt2 MW ln(MH) which were initially used to indirectly determine mt. After the top quark discovery, use precision measurements of MW and mt to constrain MH. 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 3 Top production At √s=1.96 TeV top is produced in pairs via quarkantiquark annihilation 85% of the time, gluon fusion accounts for 15% of ttbar production 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 4 Top Lifetime and Decay • Since the top lifetime the top quark does not top ~ 1/ M3top~10 -24 sec hadronize. It decays as a qcd ~ -1 ~10 -23 sec free quark! • BR(tWb) e-e (1/81) – Both W’s decay via W l mu-mu (1/81) • final state: llbb - tau-tau (1/81) DILEPTON e -mu (2/81) – One W decays via Wl • final state: lqq bb e -tau (2/81) mu-tau (2/81) - LEPTON+JETS – Both W’s decay via Wqq • final state: qq qq bb e+jets (12/81) mu+jets(12/81) tau+jets(12/81) jets ALL HADRONIC (36/81) Lepton provides a good trigger, all jets are tough 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 5 Top ID in “lepton+jets” channel • 2 b-jets pp tt • Lepton: electron or muon t bW • Neutrino (from energy imbalance) W qq'or W l • 2 q’s – transform to jets of particles • Note that these two jets come from a decay of a particle with well measured mass – W-boson – built-in thermometer for jet energies 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 6 DØ detector • Electrons are identified as clusters of energy in EM section of the calorimeter with tracks pointing to them • Muons are identified as particles passing through entire detector volume and leaving track stubs in muon chambers. Track in the central tracking system (silicon+SciFi) is matched to track in muon system • Jets are reconstructed as clusters of energy in calorimeter using cone algorithm DR<0.5 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 7 Top mass using matrix element method in Run I • Method developed by DØ (F. Canelli, J. Estrada, G. Gutierrez) in Run I Single most precise measurement of top mass in Run I Mt =180.1±3.6(stat) ±4.0(syst) GeV/c2 Systematic error dominated by JES 3.3 GeV/c2 With more statistics it is possible to use additional constraint on JES based on hadronic W mass in top events – in situ calibration 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 8 Matrix element method • Goal: measure top quark mass • Observables: measured momenta of jets and leptons • Question: for an observed set of kinematic variables x what is the most probable top mass • Method: start with an observed set of events of given kinematics and find maximum of the likelihood, which provides the best measurement of top quark mass • Our sample is a mixture of signal and background Pevt ( x, mt ) f top Psgn ( x, mt ) (1 f top ) Pbkg ( x) 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 9 Matrix Element Method probability to observe a set of kinematic variables x for a given top mass dnσ is the differential cross section Contains matrix element squared W(x,y) is the probability that a parton level set of variables y will be measured as a set of variables x 1 n Psgn ( x; mt ) d ( y; mt ) dq1 dq2 f (q1 ) f (q2 ) W ( x, y ) (mt ) f(q) is the probability distribution than a parton will have a momentum q Normalization depends on mt Includes acceptance effects Integrate over unknown q1,q2, y q b q’ t t 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 10 Transfer functions (partonjet) • Partons (quarks produced as a result of hard collision) realize themselves as jets seen by detectors – Due to strong interaction partons turn into parton jets – Each quark hardonizes into particles (mostly p and K’s) – Energy of these particles is absorbed by calorimeter – Clustered into calorimeter jet using cone algorithm • Jet energy is not exactly equal to parton energy – Particles can get out of cone – Some energy due to underlying event (and detector noise) can get added – Detector response has its resolution • Transfer functions W(x,y) are used to relate parton energy y to observed jet energy x 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 11 h Dependence of JES hdependence of JES is derived on g+jet data, but the overall scale is allowed to move to optimize MW 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 12 JES in Matrix Element • All jets are corrected by standard DØ Jet energy scale (pT, h) • Overall JES is a free parameter in the fit – it is constrained in situ by mass of W decaying hadronically • JES enters into transfer functions: W ( E j , E p , JES ) 07/22/05 W( Ej , Ep ) JES JES Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 13 Normalization 1 n Psgn ( x; mt ) d ( y; mt ) dq1 dq2 f (q1 ) f (q2 ) W ( x, y ) (mt ) e+jets 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL μ+jets 14 Signal Integration • Set of observables – momenta of jets and leptons: x • Integrate over unknown – Kinematic variables of initial (q1,q2) and final state partons (y: 6 x3 p) = 20 variables – Integral contains 15 (14) -functions for e(m)+jets • total energy-momentum conservation: 4 • angles are considered to be measured perfectly: 2x4 jet +2 lepton • Electron momentum is also considered perfectly measured, not true for muon momentum: 1(0) – 5(6) dimensional integration is carried out by Vegas – The correspondence between parton level variables and jets is established by transfer functions W(x,y) derived on MC • for light jets (from hadronic W decay) • for b-jets with b-hadron decaying semi-muonically • for other b-jets • Approximations – LO matrix element – qqtt process only (no gluon fusion – 15%) 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 15 Background integration • W+jets is the dominant background process • Kinematics of W+jets is used as a representation for overall background (admixture of multijet background is a source of systematic uncertainty) – Contribution of a large number of diagrams makes analytical calculation prohibitively complex – Use Vecbos • Evaluate MEwjjjj in N points selected according to the transfer functions over phase space • Pbkg- average over points 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 16 Sample composition Lepton+jets sample – – – – Isolated e (PT>20GeV/c, |h|<1.1) Isolated m (PT>20GeV/c, |h|<2.0) Missing ET>20 GeV Exactly four jets PT>20GeV/c, |h|<2.5 (jet energies corrected to particle level) Use “low-bias” discriminant to fit sample composition – Used for ensemble testing and normalization of the background probability. – Final fraction of ttbar events is fit together with mass e+jets m+jets # of events 70 80 Signal fraction 45±12% 29±10% 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 17 Calibration on Full MC lepton+jets 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 18 Mt=169.5±4.4 GeV/c2 JES=1.034±0.034 calibrated calibrated DØ RunII Preliminary expected: 36.4% 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 19 Systematics summary Source of uncertainty Effect on top mass (GeV/c2) B-jet energy scale +1.32-1.25 Signal modeling (gluons rad) 0.34 Background modeling 0.32 Signal fraction +0.5-0.17 QCD contribution 0.67 MC calibration 0.38 trigger 0.08 PDF’s 0.07 Total +1.7-1.6 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 20 ● B-jet energy scale Relative data/MC b/light jet energy scale ratio •fragmentation: +-0.71 GeV/c2 different amounts of p0, different p+ momentum spectrum fragmentation uncertainties lead to uncertainty in b/light JES ratio compare MC samples with different fragmentation models: Peterson fragmentation with eb=0.00191 Bowler fragmentation with rt=0.69 •calorimeter response: +0.85 -0.75 GeV/c2 uncertainties in the h/e response ratio + charged hadron energy fraction of b jets > that of light jets corresponding uncertainty in the b/light JES ratio •Difference in pT spectrum of b-jets and jets from W-decay: 0.7 GeV/c2 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 21 Gluon radiation • The effect is reduced by – Requiring four and only four jets in the final state – High PT cut on jets • Yet in ~20% of the events there is at least one jet that is not matched (DR(parton-jet)<0.5) to top decay products – These events are interpreted as background by ME method • We study this systematic by examining ALPGEN ttj sample and varying its relative fraction between 0 and 30% (verified on our data by examining the fraction of events with the 5th jet) • Final effect on top mass 0.34 GeV/c2 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 22 Signal/Background Modeling ● QCD background: +-0.67 GeV/c2 Rederive calibration including QCD events from data (lepton anti-isolation) (note: sample statistics limited) can be reduced in the future ● W+jets modeling: +-0.32 GeV/c2 study effect of a different factorization scale for W+jets events (<pT,j>2 instead of mW2 + SpT,j2) PDF uncertainty: +-0.07 GeV/c2 ● CTEQ6M provides systematic variations of the PDFs reweight ensembles to compare CTEQ6M with its systematic variations (by default the measurement uses CTEQ5L throughout: use a LO matrix element, and for consistency with simulation) 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 23 ● Signal fraction Signal fraction: +0.50 -0.17 GeV/c2 Fitted top mass depends slightly on true signal fraction (if signal fraction is smaller than expected): => Vary signal fraction within uncertainties from topological likelihood fit - Note: ftop fit yields identical result with factor √2 smaller uncertainties Cross check on data: cut on log10(pbkg)<-13 Ftop=31%46±6% Mtop=170.2±4.1 GeV/c2 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 24 Systematics summary Source of uncertainty Effect on top mass (GeV/c2) B-jet energy scale +1.32-1.25 Signal modeling (gluons rad) 0.34 Background modeling 0.32 Signal fraction +0.5-0.17 QCD contribution 0.67 MC calibration 0.38 trigger 0.08 PDF’s 0.07 Total +1.7-1.6 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 25 Result and cross checks • Run II top quark mass based on lepton+jets sample: Mt=169.5 ±4.4(stat+JES) +1.7-1.6 (syst) GeV/c2 • JES contribution to (stat+JES) 3.3 GeV/c2 • Break down by lepton flavor – Mt(e+jets)=168.8 ±6.0(stat+JES) GeV/c2 – Mt(m+jets)=172.3 ±9.6(stat+JES)GeV/c2 • Cross check W-mass 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 26 Summary of DØ Mt measurements • Statistical uncertainties are partially correlated for all l+jets Run II results 07/22/05 DØ Run II preliminary Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 27 Projection for uncertainty on top quark mass Assumptions: • only lepton+jets channel considered • statistical uncertainty normalized at L=318 pb-1 to performance of current analyses. • dominant JES systematic is handled ONLY via in-situ calibration making use of MW in ttbar events. • remaining systematic uncertainties: include b-JES, signal and background modeling, etc (fully correlated between experiments) Normalized to 1.7 GeV at L=318 pb-1. • Since most of these systematic uncertainties are of theoretical nature, assume that we can use the large data sets to constrain some of the model parameters and ultimately reduce it to 1 GeV after 8 fb-1. 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 28 Combination of Tevatron results JES is treated as a part of systematic uncertainty, taken out of stat error 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 29 Combination • Mt=172.7±2.9 GeV/c2 • Stat uncertainty: 1.7GeV/c2 • Syst uncertainty: 2.4GeV/c2 • hep-ex/0507091 • Top quark Yukawa coupling to Higgs boson • gt=Mt√2/vev=0.993±0.017 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 30 What does it do to Higgs? MW,GeV/c2 68% CL MH,GeV/c2 Mt,GeV/c2 • MH=91+45-32GeV/c2 • MH<186 GeV/c2 @95%CL 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 31 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 32 ttbar cross section in l+jets with b-tag DØ RunII Preliminary, 363pb-1 • Isolated lepton – pT>20 GeV/c, |he|<1.1, |hm|<2.0 • Missing ET>20GeV • Four or more jets – pT>15 GeV/c, |h|<2.5 =8.1+1.3-1.2(stat+syst)±0.5(lumi) pb ≥4j, 1t ≥4j, 2t Expect bkg 21.8±3.0 1.9±0.5 Observe 88 07/22/05 21 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 33 Cross section summary DØ RunII Preliminary Submitted for publication Updates ( pp tt ), pb 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 34 ttbar resonances in l+jets with b-tag • Check ttbar invariant mass for possible resonance production DØ RunII Preliminary, 363pb-1 NNLO(tt)=6.77±0.42 • Events are kinematically constrained – mT=175GeV/c2 – Leptonic and hadronic W masses 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 35 ttbar resonances in l+jets with b-tag • Limit M(Z’)>680 GeV/c2 with G=1.2%MZ’ at 95%CL DØ RunII Preliminary, 363pb-1 * *R. Harris, C. Hill, S. Parke hep-ph/9911288 Run I limit 560 GeV/c2 Run II limit 680 GeV/c2 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 36 Conclusion • First DØ RunII top mass measurement in l+jets channel to surpass Run I precision – Mt=169.5 ±4.4(stat+JES) +1.7-1.6 (syst) GeV/c2 • Developed method for in situ jet energy scale calibration using hadronic W-mass constraint • Combined Tevatron top mass measurement reaches a precision of 1.7% • ttbar production cross sections updated for l+jets channel • Invariant mass of ttbar system probed for resonance production, exclusion limit for M(Z’)>680 GeV/c2 at 95%CL 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 37 Backup slides Parton Level Tests Text 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 39 L+jets sample composition 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 40 Kinematics in l+jets sample DØ RunII Preliminary, 363pb-1 07/22/05 Regina Demina, Joint Theoretical and Experimental Seminar at FNAL 41